1. Technical Field
The present invention relates to isolation technologies for semiconductor integrated circuit devices. More particularly, the present invention relates to a method for forming void-free trench isolation layer by using He gas in high density plasma chemical vapor deposition (HDP-CVD) equipment.
2. Description of Related Arts
Shallow trench isolation (STI) is one of the most widely used technologies for insulating or isolating individual elements in semiconductor integrated circuits. The isolation region or layer formed by the STI occupies smaller area while providing good isolation quality and is thus suitable for higher integration of the IC devices.
As the degree of integration increases, it is beneficial to form a void-free isolation layer filled with oxide material in a shallow trench having higher aspect ratio equal to or greater than 3.
Voids may occur in STI processes due to a lack of process margin, which results in yield losses, such as scrapping of lots of wafers, and possibly a need to re-work the wafers. For solving this problem, other fabrication processes could be developed to secure enough process margin. However, known scheme(s) may be difficult and/or not cost effective. Therefore, it is desired to obtain broader margin by modifying the trench isolation process.
In the trench isolation formation process, HDP-CVD equipment is used to form an oxide film in the shallow trench. The oxide film is deposited by the combination of deposition and simultaneous sputtering of reaction gases in HDP-CVD system. Specifically, a gas mixture of SiH4 and O2 are ionized and/or converted to reactive species such as radicals by a high-density plasma source, and the ions and/or other reactive species form SiOx. Here, O2 contributes to the deposition of the oxide layer and at the same time contributes to sputtering by the bias power applied to the reaction chamber.
During the formation of the oxide film in the HDP-CVD equipment, the ratio of deposition and sputtering is denoted by ‘D/S’ (“deposition-to-sputter-etching ratio” or simply “sputter-etch ratio”), which is defined as “(net deposition rate+blanket sputtering rate)/(blanket sputtering rate)”. The value of D/S can determine the gap-fill capability of HDP-CVD process. When D/S is large, the deposition rate is greater than the sputter etching rate, and the formation of oxide layer is largely depending on deposition rather than sputter etching by the reaction gases. In the mean time, a lower D/S indicates that sputter etching by the reaction gases dominates or plays a relatively important role in the formation of the oxide layer. Therefore, a lower D/S is advantageous for filling a trench having an aspect ratio of 3 or greater, because the sputter etching may prevent the trench hole from closing before complete gap-filling. However, if the ratio (D/S) is too small, trench patterns can be etched back or sputtered off. Therefore, it is significant to choose the proper value of D/S according to the processing conditions.
A conventional trench isolation is formed in HDP-CVD equipment using SiH4 and O2 for the oxide formation gases. However, when molecules having large atomic radius such as O2 are used, the sputtered ions from the O2 atoms collide with the reaction gases and may cause re-deposition of oxide at the sidewalls in upper corners of the trench. Thus, formation of oxide films without voids may be difficult when a trench having a width less than 150 nm is filled using O2 gas. Further, trench isolation structures formed using O2 have a greater difference in the wave height, and hence wafer defects in subsequent processes such as chemical mechanical polishing (CMP) process may be produced.